When a shallow doped layer or region, such as a boron layer, is formed in a semiconductor substrate by ion implantation, the junction depth is not just dependent on the ion implant energy but can also depend on channeling and phenomena such as transient enhanced diffusion (TED) when the implanted ions migrate through the crystal lattice during subsequent thermal processing. Current techniques for forming ultra-shallow doped regions, such as PLDD regions in CMOS devices, use pre-amorphisation techniques to amorphise the semiconductor substrate (i.e., turn a portion of the crystalline silicon substrate into amorphous silicon) by, for example, ion implantation using non-electrically active ions, such as silicon, germanium and fluorine, in order to eliminate channeling. The pre-amorphisation implantation creates in the substrate an amorphous surface layer adjacent to the underlying crystalline semiconductor material and produces a large number of defects beyond the amorphous/crystalline (a/c) interface. These crystal defects are usually called End of Range (EOR) defects. Defects of this kind are known to enhance diffusion of previously implanted dopant ions during subsequent thermal processes of annealing and activation of the semiconductor device. It is also known that during the heat treatment (for annealing and activation), the amorphised layer re-crystallizes and the EOR defects dissolve semiconductor interstitials that effectively migrate towards the surface of the structure, so that they become present in the surface doped layer to provide a mechanism for TED.
TED increases the diffusivity of the dopant in the doped layer with the result that the depth of the shallow doped layer is increased beyond its target depth. With the desire to reduce the size of semiconductor devices, several techniques have been proposed to reduce the effects of TED so as to reduce the depth of the doped layer by reducing the EOR defects.
One such technique provides a layer rich in a trap element located between a surface implanted boron layer, and the EOR defects beyond the amorphous/crystalline (a/c) interface. Then, during heat treatment, migrating defects are essentially halted or trapped by this layer and prevented from migrating up to the surface to provide the TED mechanism in the boron layer. As a result, a junction can be formed in the substrate that is shallower and can have a steeper profile.
In a typical process, a pre-amorphisation implantation of Ge is followed by a carbon (C) implantation to form a trapping layer for preventing interstitial back flow. Fluorine (F) is then implanted to address Negative Bias Temperature Instability (NBTI) or for dopant activation purposes. Then, Nitrogen (N) is implanted to help prevent boron (B) deactivation. Finally, B is implanted into the substrate and activated by a solid phase epitaxy (SPE) anneal.
FIG. 1 illustrates this process and shows the implanted C layer trapping the EOR defects during the post-B implantation anneal.
This conventional method of forming shallow junctions and addressing TED issues associated therewith necessarily involves at least four separate implantation steps (i.e., Ge pre-amorphisation implantation, C implantation, F implantation and N implantation) before the B implantation. This multistep process has time and expense cost penalties. Further, there are penalties associated with the Ge, N, C and F impurities, such as sheet resistance (Rsd) degradation and junction leakage. Moreover, impurities such as N and C have hot carrier and NBTI penalties.
An implantation procedure for forming ultra-shallow doped regions with reduced TED is desired.